The present invention relates to a cutting insert which is ground to form a sharp edge. The cutting insert is primarily intended for milling, although also cutting inserts for turning and drilling lie within the framework of the invention.
Cutting inserts such as indexable inserts are made by form-pressing and sintering of a cutting material-forming powder, usually cemented carbide. The cutting insert comprises (a) an upper surface or chip surface, (b) a bottom surface substantially parallel with the upper surface (these two surfaces constitute the main surfaces) and abuttable against a cooperating bottom support surface in an insert seat, and (c) side or relief surfaces extending between these upper and bottom surfaces. At least one of the side or relief surfaces is abuttable against a cooperating side support surface in the insert seat. The side surfaces may be perpendicular to said upper and bottom surfaces, but usually they are somewhat obliquely positioned, i.e., oriented at an acute angle to the upper surface, and at an obtuse angle to the bottom surface. A transition between the upper surface and a side surface is shaped as a cutting edge.
In production of primarily indexable cutting inserts of cemented carbide, a direct pressing method is being employed to an increasing extent, according to which method a cemented carbide forming powder first is shaped into the desired form in a pressing tool, and thereafter it is given the final strength by being sintered in an oven. The pressing operation as such has been refined and is today so rational that it brings about good possibilities of forming the cutting edge and the appurtenant chip shaping surfaces and possible reinforcing faces with great accuracy.
For some types of machining, e.g., some forms of face milling and corner milling, the requirements of shape and dimensional accuracy of the inserts have been raised during the last years. Primarily geometries with sintered-in positive cutting edges require a very high dimensional accuracy in order to guarantee a satisfactory result at small tooth feeds. These requirements of dimensional accuracy, and in some cases also on sharp cutting edges, have hitherto been achieved by so-called contour grinding or peripheral grinding, wherein the surface(s) that adjoins the individual cutting edge is after-ground following the usual grinding and cooling. This contour grinding is made on the relief side of the cutting insert, i.e., on the side surfaces. As an example of such a grinding, U.S. Pat. No. 5,078,550 is referred to. However, thereby the working measure or the IC measure are altered. Further, when the cutting insert has sintered-in chip breakers, the after-grinding may result in the primary land of the cutting insert becoming unacceptably narrow and fragile. Moreover, the bottom surface of the cutting inserts have been ground a substantially perfect plane, in order to guarantee a safe and stable support in the insert seat.
However, in order to not excessively alter the working measure of the cutting insert and at the same time obtain a sharp cutting edge, it would be desirable to be able to grind the cutting edge with the appurtenant primary land from above, i.e., on the upper or chip surface of the cutting insert. However, in conventional grinding machines this has not been feasible due to the fact that very little material of the upper surface is to be ground, i.e., only the cutting edge line or a primary land immediately adjacent the cutting edge line. Because of the small extent or area of the cutting edge line or primary land (often &lt;0.2 mm), the speed of the grinding plate has to be lowered and then undesired chippings are obtained. Furthermore, it is extremely difficult to control the grinding capacity when there is not enough material that exerts a resistance against the grinding operation, whereby the precision of the ground face becomes poor.
Thus, a primary object of the present invention is to provide a cutting insert that may be ground with a substantially unaltered chip cutting geometry on the rake side as well as on the relief side by a conventional grinding mechanism.
A second object of the invention is to be capable of producing sharp cutting edges without altering the chip cutting geometry.
Still another object of the present invention is to rationalize the grinding of the cutting inserts as much as possible.